The Mediterranean Sea urchins
Classification
- Asteroidea (the starfish),
- Crinoidea (the crinoids),
- Ophiuroidea (the brittle stars),
- Holothuridea (the sea cucumbers),
- Echinoidea (the sea urchins, indeed).
- pentaradial symmetry, meaning their body is divided into five mostly symmetrical “sectors.” Of course, there are exceptions (sea cucmbers, some starfish, irregular urchins).
- endoskeleton, that is, a skeleton that acts as a “skin” made up of calcareous plates. These plates can be fused, making the skeleton rigid as in sea urchins, or movable, thus creating a flexible skeleton as in starfish.
- water vascular system, which is an internal sea water circulation system that allows them, among other things, to move.
- pronounced regenerative capacity, which allows them to rebuild some of their parts after damage. Starfish, for example, can regrow a limb if severed, or sea urchins can regenerate parts of the endoskeleton if broken.
- regular urchins: urchins in which the mouth is on the underside and the anus is symmetrically on the upper side; they live on hard substrates (rocks, coralline).
- irregular urchins: urchins in which the mouth and anus are no longer symmetrical but have shifted respectively forward and backward; they normally live buried in sand or mud. An example is the spatango (Spatangus purpureus) in Fig. 10.
Internal and external structure
A general cross-section of the structure of a sea urchin is shown in Fig. 3, along with an external detail in the photo in Fig. 4.
The endoskeleton (see Figs. 1 and 2), also called the test, is the rigid external structure, roughly spherical in shape. It is rigid because it is formed by calcite plates fused together.
At the center of the lower or ventral side is the mouth, which is in contact with the substrate and consists of the so-called Aristotle’s lantern, a structure made up of five (!) extremely hard teeth that the urchin uses to grind algae or other relatively hard prey.
On the upper side, called the apical area, around the anus, there are five genital plates. These plates have a so-called gonopore, through which gametes are released during reproduction. One of these plates, known as the madreporite, contains an opening through which seawater is introduced into the urchin via the water vascular system. This system is responsible for the movement of the ambulacral feet and thus enables locomotion. (see this interesting article for further details).
Throughout the endoskeleton, there are several appendices of varying length and rigidity, serving different functions:
- spines, rigid and pointed extensions that, in addition to being a clear defensive weapon, also contribute to locomotion.
- podia or ambulacral feet, equipped with a suction cup at the end, which extend outward for locomotion (this movement is facilitated by the water vascular system, which connects the podia to the madreporites).
pedicellariae, generally shorter than the podia, are necessary for touch and respiration.
Reproduction
Sea urchins generally have separate sexes (this is referred to as gonochorism). Males and females, usually indistinguishable by appearance, release their gametes synchronously (or at least, that is the intention!) when environmental conditions are favorable. This is one of nature’s miracles: how do organisms like echinoderms, or even corals, manage to release gametes simultaneously without seemingly communicating with one another?! The fertilization of the eggs occurs in open water. Larvae are then formed, which drift in the plankton until they are mature enough to find a spot on the seabed and begin their adult life.
Incredibly, the larvae do not exhibit pentaradial symmetry but instead bilateral symmetry (the body is divided into two symmetrical halves)! In Fig. 5, you can see how the larva of the sea urchin, the pluteus (a microscopic image shown in Fig. 6), undergoes a radical transformation during its development before settling and becoming an adult sea urchin.
The release of gametes, known as spawning, typically occurs once a year. However, the timing varies significantly depending on the context. The usual influencing factors are hydrodynamics, temperature, the presence of nutrients in the water, and sunlight exposure.
Lastly, you’ve likely already realized that when we eat sea urchins, it is actually their gonads (i.e., eggs and sperm) that we consume. But if you think about it, this is true for many edible marine products…
The most common species in the Med
Purple sea urchin (Paracentrotus lividus) and black sea urchin (Arbacia lixula)
Purple sea urchin
(Paracentrotus lividus)
&
Black sea urchin
(Arbacia lixula)
- Echinodermata
- Echinoidea
- Camarodonta
- Parechinidae
- Paracentrotus
- P. lividus
- Echinodermata
- Echinoidea
- Arbacioida
- Arbaciidae
- Arbacia
- A. lixula
Let’s clarify right away that these are two distinct species and not, as many believe, the female and male of the same species!
The main difference lies in their external coloration: A. lixula (Fig. 8) is jet black, while P. lividus (Fig. 7) can exhibit a variety of colors ranging from light brown to intense purple or fuchsia.
Both primarily feed on algae, with A. lixula especially favoring encrusting calcareous algae. Additionally, neither species particularly likes light, though A. lixula tolerates it better (fun fact: P. lividus often partially shields itself by attaching small pieces of algae or pebbles to its shell, a behavior never observed in A. lixula).
P. lividus is the quintessential edible sea urchin, prized for its gonads (the five “sacs” containing eggs in females and sperm in males, arranged in a radial pattern at the top of the urchin), which are particularly enjoyed by Sardinians and Sicilians. A. lixula, on the other hand, is not edible.
Beyond human predation, both species are preyed upon by certain fish. When they are small, wrasses (Coris julis, Thalassoma pavo) can break their shells, while adult urchins are still vulnerable only to larger fish such as the white seabream (Diplodus sargus) and the two-banded seabream (Diplodus vulgaris) (Sala and Zabala, 1986).
Some crustaceans as well occasionally feed on both types of urchins (Eriphia verrucosa, Maja squinado, or even the spiny lobster Palinurus elephas).
Two less obvious predators are instead the spiny starfish (Marthasterias glacialis) and the banded murex (Hexaplex trunculus).
The spiny starfish, that is, a close relative of the urchins, envelops the prey as shown in Fig. 9, flips it over, and, by everting its stomach, first digests the external tissues of the urchin near the Aristotle’s lantern, then penetrates it to feed on its “filling” (Guidetti, 2004). Typically, the unfortunate urchin is left with a semi-bare shell, with a few spines still attached that soon fall off. In some cases, the starfish’s grip is so forceful that the shell breaks into several pieces!
Even more bizarre is the predation by the banded murex, a gastropod mollusk. This predator is capable of drilling through the urchin’s shell with its proboscis and emptying the urchin from the inside using its radula. From now on, if you find intact sea urchin shells (tests) with a small hole no larger than 1 cm, you’ll know who feasted on them!
Violet sea urchin (Sphaerechinus granularis)
Violet sea urchin
(Sphaerechinus granularis)
- Echinodermata
- Echinoidea
- Camarodonta
- Toxopneustidae
- Sphaerechinus
- S. granularis
Identification is easy based on its spines: they vary in color but are always short and white at the tips (or, in some cases, entirely white).
The violet sea urchin primarily feeds on algae, like the purple and black sea urchins, but does not disdain dead leaves of posidonia. Similar to purple sea urchins, it often “decorates” itself with small pieces of algae or pebbles, which is actually a way to protect itself, at least partially, from light!
Hatpin sea urchin (Centrostephanus longispinus)
Hatpin sea urchin
(Centrostephanus longispinus)
- Echinodermata
- Echinoidea
- Diadematoida
- Diadematidae
- Centrostephanus
- C. longispinus
Its unmistakable feature is its spines, which are extremely long on the upper part and black (in juveniles, they have light and dark stripes). Thus its other common name, long-spined sea urchin.
This sea urchin does not like light at all; during the day, you will always find it at the entrance of caves or in narrow, dark crevices. At night, however, it becomes bold and ventures out of its daytime refuge to feed.
On the apical area of the test, there are some modified podia whose function has not yet been determined. As seen in the video below, these end in a fluorescent purple ball and rotate in a psychedelic fashion, some clockwise and others counterclockwise.
To us, it’s like the Mystery of the Seven Seas, captivating us every time we observe it (Doris FFESSM, 2024)!
Pencil sea urchin (Stylocidaris affinis)
Pencil sea urchin
(Stylocidaris affinis)
- Echinodermata
- Echinoidea
- Cidaroida
- Cidaridae
- Stylocidaris
- S. affinis
A truly unique and, in our opinion, extremely fascinating sea urchin, it has only a few but very thick spines. Its color varies from gray to orange, and unlike the previously mentioned urchins, it is found at greater depths (around 30 meters).
The spines are irritating to humans, and they are often found broken among fishermen’s nets (or nearby) left in the sun. Be careful not to approach them barefoot!
Fun fact: in the Mediterranean, there are actually two species of pencil urchin, Stylocidaris affinis and Cidaris cidaris. Differentiating them with certainty requires a microscope, as there is still no consensus on how to distinguish them with the naked eye. Some say one has longer or lighter spines, or spines with clearer bands alternating with darker ones, while others claim that one species is herbivorous, and the other prefers gorgonians (Mastrototaro et al., 2021): yet another mystery!
Melon sea urchin (Echinus melo)
Melon sea urchin
(Echinus melo)
- Echinodermata
- Echinoidea
- Camarodonta
- Echinidae
- Echinus
- E. melo
This is a sea urchin that can be quite big (up to almost 20 cm in diameter), with a light color tending toward greenish, and a test that is almost perfectly spherical rather than flattened like all the previously mentioned urchins. Its spines are very sparse, divided into primary spines, which are longer, and secondary spines, which are significantly shorter.
Perhaps our favorite… not least because it is generally found at even greater depths than the pencil urchin, always beyond 40 meters, and very often inside semi-dark caves – all features we find particularly intriguing!
Spatangus purpureus
- Echinodermata
- Echinoidea
- Spatangoida
- Spatangidae
- Spatangus
- S. purpureus
Not liking light at all, like all its irregular sea urchin cousins, it is observed almost exclusively at night (and even then, rarely) because during the day it lives completely buried in sand or mud.
It is therefore easier to find its shell, on the sand, devoid of its short and dense spines. Its shape never fails to surprise us: resembling a flattened heart, with symmetry no longer pentaradial but bilateral (two identical longitudinal halves), it features a peculiar starfish pattern imprinted on its upper surface (similar to the one in Fig. 2)!
This pattern serves as proof that pentaradial symmetry existed at some point in time, as it is only since the Jurassic period that irregular sea urchins have evolved, differentiating themselves from regular ones.
The strange case of the mortality of the invasive long-spined sea urchin Diadema setosum
- Echinodermata
- Echinoidea
- Diadematoida
- Diadematidae
- Diadema
- D. setosum
In 2006, a long-spined sea urchin endemic of the Red Sea, Diadema setosum, was observed for the first time in Turkey. Since then, it has rapidly spread throughout the Levantine Basin of the Med, where it is now commonly seen (for instance, it is everywhere in the Aegean Sea). It has adapted well here, and who can blame it!
However, in July 2022, in the southeastern Aegean, incidents of mass mortality were observed, likely due to the presence of an externally introduced pathogen (Zirler et al., 2023).
This might not seem like a serious issue, considering that it is, after all, an invasive species in the Mediterranean. But the problem is that starting from the Med, the pathogen made its way into the Red Sea, where the sea urchin is indeed endemic! In fact, beginning in autumn 2022, mass die-offs were reported in the northern Red Sea, particularly in the Gulf of Aqaba, where D. setosum has completely disappeared in some areas. Ah, that cursed Suez Canal!
References
A. Baucon, “Corso di Paleontologia Università degli studi di Trieste”, https://www.tracemaker.com/paleontologia-palaeontology/
P. Guidetti, “Consumers of sea urchins, Paracentrotus lividus and Arbacia lixula, in shallow Mediterranean rocky reefs”, Helgoland Marine Research, 58, 2004
F. Mastrototaro, F. Montesanto, A. Tursi, R. Aguilar, G. Chimienti, “Biometry supporting species identification from underwater images in two Mediterranean Sea urchins”, 2021 International Workshop on Metrology for the Sea; Learning to Measure Sea Health Parameters (MetroSea), 2021
E. Sala, M. Zabala, “Fish predation and the structure of the sea urchin Paracentrotus lividus populations in the NW Mediterranean”, Marine Ecology Progress Series, 140, 1996
M. L. Vittelletti, “Biologia riproduttiva di Paracentrotus lividus (Lamarck, 1816) in due zone della Sardegna”, PhD thesis, 2013
Website “Doris” of the Fédération Française d’Études et de Sports Sous-Marins, https://doris.ffessm.fr/Forum/Fonction-des-radioles-mauve-dans-l-oursin-diademe-mediterraneen-82928
Zirler et al., “Mass mortality of the invasive alien echinoid Diadema setosum (Echinoidea: Diadematidae) in the Mediterranean Sea”, Royal Society Open Science, 10, 2023
In which itinerary do we spot it?
- San Vito, Ustica
San Vito, Ustica - Favignana, Levanzo, MarettimoFavignana, Levanzo, Marettimo
- Filicudi, Lipari, Capo MilazzoFilicudi, Lipari, Capo Milazzo
- Lampedusa, LinosaLampedusa, Linosa
